The present invention relates to an electrode for a capacitor suitable for use in a semiconductor memory cell in particular, said capacitor and a process for the manufacture of these.
A nonvolatile semiconductor memory cell using a ferroelectrics film (to be sometimes referred to as xe2x80x9cFERAMxe2x80x9d hereinafter) is a nonvolatile semiconductor memory cell which uses fast inversion of polarization of the ferroelectrics film and residual polarization thereof and permits rapid rewriting. A nonvolatile semiconductor memory cell having a ferroelectrics film under study at present can be classified into two memory cells, a memory cell based on a method of detecting a change in an amount of a stored charge on a ferroelectrics capacitor and a memory cell based on a method of detecting a change in resistance of a semiconductor caused by the spontaneous polarization of ferroelectrics. The semiconductor memory cell to be explained hereinafter in the present specification comes under the former, and it is, in principle, composed of a ferroelectrics capacitor and a selecting transistor and has a structure and constitution similar to those of DRAM.
In the nonvolatile semiconductor memory cell according to the method of detecting a change in the amount of the stored change on the ferroelectrics capacitor, data is written and read out by applying the P-E hysteresis loop of ferroelectrics shown in FIG. 11. When an external electric field is applied to a ferroelectrics film and then removed, the ferroelectrics film exhibits spontaneous polarization. The residual polarization of the ferroelectrics film is +Pr when an external electric field in a plus direction is applied, and it is xe2x88x92Pr when an external electric field in a minus direction is applied. In this case, a state where the residual polarization is +Pr (see xe2x80x9cDxe2x80x9d in FIG. 11) represents xe2x80x9c0xe2x80x9d, and a state where the residual. polarization is xe2x88x92Pr (see xe2x80x9cAxe2x80x9d in FIG. 11) represents xe2x80x9c1xe2x80x9d.
For discriminating xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d,for example, an external electric field in a plus direction is applied to the ferroelectrics film. As a result, the polarization of the ferroelectrics film is brought into a state xe2x80x9cCxe2x80x9d in FIG. 11. When data is xe2x80x9c0xe2x80x9d the polarization state of the ferroelectrics film changes from xe2x80x9cDxe2x80x9d to xe2x80x9cCxe2x80x9d. On the other hand, when data is xe2x80x9c1xe2x80x9d, the polarization state of the ferroelectrics film changes from xe2x80x9cAxe2x80x9d to xe2x80x9cCxe2x80x9d through xe2x80x9cBxe2x80x9d. When data is xe2x80x9c0xe2x80x9d, the ferroelectrics film causes no inversion of the polarization. On the other hand, when data is xe2x80x9c1xe2x80x9d, the ferroelectrics film causes an inversion of the polarization. As a result, a difference is caused in the amount of transferred charge depending upon a difference in the stored charge amount (polarization state) of the ferroelectrics capacitor. The stored charge is detected as a signal current by turning on the selecting transistor of a selected semiconductor memory cell. When the external electric field is changed to xe2x80x9c0xe2x80x9d, the polarization state of the ferroelectrics film is brought into a state xe2x80x9cDxe2x80x9d in FIG. 11 even when the data is any one of xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d. When the data is xe2x80x9c1xe2x80x9d, therefore, the polarization is brought into a state xe2x80x9cAxe2x80x9d through xe2x80x9cDxe2x80x9d and xe2x80x9cExe2x80x9d by applying an external electric field in a minus direction, to write data xe2x80x9c1xe2x80x9d.
The dielectric capacitor of the semiconductor memory cell is composed of two electrodes and a capacitor insulation layer formed of a dielectric film sandwiched between these two electrodes. In a conventional nonvolatile semiconductor memory cell (FERAM) using a ferroelectrics film, an upper electrode and a lower electrode are formed of platinum (Pt). Platinum is a stable material, while it has a defect that its processability is poor. It is therefore being studied to use Ru, RuO2, Ir, IrO2 or the like as an electrode material-having excellent processability over platinum, for high-density integrated DRAM or FERAM.
For stably maintaining the electric properties of a dielectric capacitor for a long period of time, desirably, the electrode in the interface with a dielectric film is composed of RuO2 or IrO2, as is discussed in xe2x80x9cImprovement of Fatigue of PZT Capacitors by Optimizing Electrode Material and PZT Crystallinityxe2x80x9d, K. Aoki, et al., PacRimFerro 3 in Kyoto, extended abstract, pp 87-90 (1996). However, it is known that when Ru is oxidized, volatile ruthenium oxides, RuO3 and RuO4, are generally formed and that when Ir is oxidized at a high temperature, it is volatilized as IrO2. When these metals are used as a lower electrode in particular, the lower electrode is exposed to a high-temperature atmosphere for a long period of time and the flatness of the lower electrode surface is impaired by evaporation and/or re-oxidation of these materials. As a result, there is caused a problem that the dielectric capacitor is degraded in characteristics.
It is therefore an object of the present invention to provide an electrode for a capacitor, a dielectric capacitor of which is not degraded in characteristics when the electrode is exposed to a high-temperature atmosphere for a long period of time, said capacitor and a process for the manufacture of these.
According to the present invention, the above object of the present invention is achieved by an electrode for a capacitor composed of two electrodes and a capacitor insulation layer formed of a dielectric film sandwiched between the two electrodes,
at least one of the electrodes being formed of a metal layer and a metal oxide layer, and
said metal oxide layer being formed by oxidizing a surface of said metal layer on the basis of a diffusion-controlling reaction and being positioned in an interface to said capacitor insulation layer.
According to the present invention, the above object of the present invention is also achieved by a process for the manufacture of an electrode for a capacitor composed of two electrodes and a capacitor insulation layer formed of a dielectric film sandwiched between the two electrodes,
The process comprising forming a metal layer, and then oxidizing a surface of said metal layer on the basis of a diffusion-controlling reaction by heat-treating said metal layer in an oxidizing atmosphere, to form a metal oxide layer, thereby forming at least one of the electrodes composed of the metal layer and the, metal oxide layer positioned in an interface to the capacitor insulation layer.
Further, according to the present invention, the above object is achieved by a capacitor composed of two electrodes and a capacitor insulation layer formed of a dielectric film sandwiched between the two electrodes,
at least one of the electrodes being formed of a metal layer and a metal oxide layer,
said metal oxide layer being formed by oxidizing a surface of said metal layer on the basis of a diffusion-controlling reaction and being positioned in an interface to said capacitor insulation layer, and
the dielectric film being formed of a dielectric material having a perovskite structure, a pseudo perovskite structure or a layer structure.
According to the present invention, the above object of the present invention is also achieved by a process for the manufacture of a capacitor composed of two electrodes and a capacitor insulation layer formed of a dielectric film sandwiched between the two electrodes,
the process comprising forming a metal layer, and then oxidizing a surface of said metal layer on the basis of a diffusion-controlling reaction by heat-treating said metal layer in an oxidizing atmosphere, to form a metal oxide layer, thereby forming at least one of the electrodes composed of the metal layer and the metal oxide layer positioned in an interface to the capacitor insulation layer.
The present invention includes an embodiment in which the electrode has a two-layered structure of a metal layer and a metal oxide layer, or an embodiment in which the electrode is composed of an aggregate of metal particles and a metal oxide layer is formed on the entire surface, or on part of surface, of each particle. It depends upon conditions of forming the metal layer whether or not the electrode is composed of an aggregate of metal particles. The metal oxide layer can be checked for its presence by SIMS (secondary ion mass spectroscopy), Auger electron spectroscopy, an X-ray small angle scattering method or a general X-ray diffraction method.
In the present invention, the formation of the metal oxide layer and the control of the metal oxide layer thickness on the basis of a diffusion-controlling reaction can be carried out by properly controlling the atmosphere, the temperature and the period of time employed for the heat treatment. As an oxidizing atmosphere in the present invention, for example, there is used an oxygen gas atmosphere having a pressure of 0.5 MPa to 0.1 kPa. The oxygen gas concentration in the atmosphere is preferably 100% by volume to 1% by volume. Further, the temperature for the heat treatment is 800 K to 1130 K, preferably 873 K to 1073 K, more preferably 873 K to 973 K. The period of time for the heat treatment can be properly set depending upon treatment conditions such as an oxygen gas concentration and an ambient (substrate) temperature and a designed thickness of the metal oxide layer, and the heat treatment is carried out, e.g., for 10 to 55 minutes.
In the present invention, preferably, the metal layer is composed of ruthenium (Ru) or iridium (Ir), or it is composed of a ruthenium alloy or an iridium alloy. The metal layer may contain an additive such as yttrium (Y) or Y2O3. The amount of yttrium (Y) is preferably in the range of from 2 to 5% in terms of a Y2O3 volume percent.
The metal layer can be formed, for example, by a sputtering method,an electron beam evaporation method or an MOCVD (metal organic chemical vapor deposition) method. Further, the patterning of the metal layer and the metal oxide layer can be carried out, for example, by a milling method or an RIE (reactive ion etching) method.
In the present invention, the metal oxide layer has a thickness (ds) of 1xc3x9710xe2x88x928 m to 1xc3x9710xe2x88x927 m (significant figure is 1 digit, 1xc3x9710xe2x88x928 mxe2x89xa6dsxe2x89xa61xc3x9710xe2x88x927 m), more preferably 1.0xc3x9710xe2x88x928 m to 1.3xc3x9710xe2x88x927 m (significant figure is 2 digits, 1.0xc3x9710xe2x88x928 mxe2x89xa6dsxe2x89xa61.3xc3x9710xe2x88x927 m). When the value of ds is in the above range, the process of forming the metal oxide layer proceeds mainly on the basis of a diffusion-controlling reaction, and a dense metal oxide layer can be formed.
In the method of the manufacture of an electrode for a capacitor or the method of the manufacture of said capacitor, provided by the present invention, preferably, the metal layer is temperature-increased up to a heat treatment temperature by a rapid temperature-raising method when the metal layer is heat-treated in the oxidizing atmosphere. That is, it is preferred to initiate the formation of the metal oxide layer by a so-called RTA method (rapid thermal annealing method). The temperature-raising rate for the metal layer is preferably at least 100 K/minute, more preferably at least 300 K/minute. When the temperature-raising rate for the metal layer is low, no dense metal oxide layer may be formed.
In the present invention, the dielectric film can be formed by a solution chemical deposition method (e.g., a sol-gel method or an MOD method), a chemical vapor deposition method (including a metal organic chemical vapor deposition method) or a physical vapor deposition method (a vacuum evaporation method including a laser abrasion method, or a sputtering method). The dielectric film can be patterned, for example, by a milling method or an RIE method. In the present invention, the dielectric film is preferably formed of a dielectric material having a perovskite structure, a pseudo perovskite structure or a layer structure. The dielectric film can be formed from a bismuth-layered-structure ferroelectrics material. The above bismuth-layered-structure ferroelectrics material comes under so-called non-stoichiometric compounds, and permits some tolerance ranges of contents in both of a metal element site and an anion (O, etc.) site. Further, not a few bismuth-layered-structure ferroelectrics materials show optimum electric characteristics when they have a composition somewhat deviated from their stoichiometric composition. The dielectric film in the present invention has the general formula,
(Bi2O2)2+(Amxe2x88x921BmO3m+1)2xe2x88x92
wherein A is at least one metal selected from the group consisting of Bi, Pb, Ba, Sr, Ca, Na, K and Cd, B is at least one metal selected from the group consisting of Ti, Nb, Ta, W, Mo, Fe, Co and Cr, and m is an integer of at least 1.
More specifically, the dielectric film in the present invention preferably contains, as a main crystal phase, a crystal phase of the formula (1),
xe2x80x83(BiX, Sr1xe2x88x92X)2(SrY, Bi1xe2x88x92Y)(TaZ, Nb1xe2x88x92Z)2Odxe2x80x83xe2x80x83(1)
wherein 0.9xe2x89xa6Xxe2x89xa61.0, 0.7xe2x89xa6Yxe2x89xa61.0, 0:xe2x89xa6Zxe2x89xa61.0 and 8.7xe2x89xa6dxe2x89xa69.3.
Otherwise, the dielectric film in the present invention preferably contains, as a main crystal phase, a crystal phase of the formula (2),
BiXSrYTa2Odxe2x80x83xe2x80x83(2)
wherein X+Y=3, 0.7xe2x89xa6Yxe2x89xa61.3 and 8.7xe2x89xa6dxe2x89xa69.3.
Further, more preferably, the dielectric film contains, as a main crystal phase, at least 85% by weight of a crystal phase of the formula (1) or (2). In the formula (1), (BiX, Sr1xe2x88x92X). means that Sr is substituted in a site for originally present Bi in a crystal structure and that the Br:Sr amount ratio is X: (1xe2x88x92X), and (SrY, Bi1) means that Bi is substituted in a site for originally present Sr in a crystal structure and that the Sr:Bi amount ratio is Y:(Yxe2x88x921). The dielectric film containing, as a main crystal phase, a crystal phase of the formula (1) or (2) may contain a Bi oxide, oxides of Ta and Nb and composite oxides of Bi, Ta and Nb to some extent.
Otherwise, the dielectric film in the present invention may contain a crystal phase of the formula (3),
BiX(Sr, Ca, Ba)Y(TaZ, Nb1xe2x88x92Z)2Odxe2x80x83xe2x80x83(3)
wherein 1.7xe2x89xa6Xxe2x89xa62.5, 0.6xe2x89xa6Yxe2x89xa61.2, 0xe2x89xa6Zxe2x89xa61.0 and 8.0xe2x89xa6dxe2x89xa610.0.
In the formula (3), (Sr, Ca, Ba)xe2x80x3 stands for one element selected from the group consisting of Sr, Ca and Ba. The dielectric film of the formula (3) has a stoichiometric composition, e.g., of Bi2SrTa2O9, Bi2SrNb2O9, Bi2BaTa2O9, or Bi2SrTaNbO9. Further, the dielectric film in the present invention may have a composition of Bi4SrTi4O15, Bi4Ti3O12 or Bi2PbTa2O9. In the above cases, the amount ratio of metals may be varied so long as the crystal structure of the dielectric film does not change. That is, the contents in both of the metal element site and oxygen element site may be varied and may be deviated from their stoichiometric composition to some extent.
Further, the material for the dielectric film includes PbTiO3, PZT [Pb(Zr1xe2x88x92y, Tiy)O3 in which 0 less than y less than 1] which is a solid solution of PbZrO3 and PbTiO3 having perovskite structures, and PZT-containing compounds such as PLZT which is a metal oxide obtained by adding La to PZT and PNZT which is a metal oxide obtained by adding Nb to PZT.
The above materials for the dielectric film come under a so-called ferroelectrics material, while the material for the dielectric film also includes high-dielectric-constant materials having a perovskite structure or a pseudo perovskite structure such as BaTiO3, SrTiO3 and (Ba, Sr)TiO3.
In the present invention, the metal oxide layer is formed by oxidizing the surface of the metal layer on the basis of a diffusion-controlling reaction, so that the dense metal oxide layer of passivity can be formed under controlled conditions. Further, the metal oxide layer of at least one of the electrodes is formed so as to be present in an interface to the capacitor insulation layer, so that the capacitor is free from the impairment of flatness of the electrode surface caused by evaporation and/or re-oxidation when the electrode is exposed to a high-temperature atmosphere for a long period of time.